Refrigeration cycle apparatus

ABSTRACT

A refrigeration cycle apparatus  100  is provided with a refrigerant circuit  106,  an injection flow passage  111,  and a high-pressure supply passage  130.  The refrigerant circuit  106  includes a low-pressure stage compressor  105,  a high-pressure stage compressor  101,  a heat radiator  102,  an expander  103,  a gas-liquid separator  108,  and an evaporator  104.  The expander  103  and the low-pressure stage compressor  105  are coupled by a power-recovery shaft  107.  The refrigeration cycle apparatus  100  is further provided with a flow passage-switching mechanism that selectively connects one of the evaporator  104  and the high-pressure supply passage  130  to the low-pressure stage compressor  105.  The flow passage-switching mechanism, for example, is constituted by an on-off valve  131  and a check valve  132.

TECHNICAL FIELD

The present invention relates to a refrigeration cycle apparatus.

BACKGROUND ART

A refrigeration cycle apparatus 700 shown in FIG. 15 is conventionallyknown as a refrigeration cycle apparatus provided with an expander thatrecovers power by expanding a refrigerant, and a second compressor thatpreliminarily increases the pressure of the refrigerant (for example,see JP 2003-307358 A). With reference to FIG. 15, the configuration ofthe conventional refrigeration cycle apparatus 700 is described.

As shown in FIG. 15, the refrigeration cycle apparatus 700 is providedwith a refrigerant circuit 6 formed of a first compressor 1, a heatradiator 2, an expander 3, an evaporator 4, a second compressor 5, andflow passages 10 a to 10 e connecting these components in this order.The second compressor 5 is coupled to the expander 3 by a power-recoveryshaft 7, and is driven by receiving, via the power-recovery shaft 7,mechanical energy recovered in the expander 3.

Further, a bypass passage 8 that bypasses the second compressor 5 and abypass valve 9 that controls the flow of the refrigerant in the bypasspassage 8 are provided therein. The upstream end of the bypass passage 8is connected to the flow passage 10 d that connects the outlet of theevaporator 4 and the suction port of the second compressor 5. Thedownstream end of the bypass passage 8 is connected to the flow passage10 e that connects the discharge port of the second compressor 5 and thesuction port of the first compressor 1.

The refrigeration cycle apparatus 700 is activated according to thefollowing procedures. First, the operation of the first compressor 1 isstarted, and the bypass valve 9 is opened. These allow the refrigerantin the evaporator 4 to be drawn into the first compressor 1 through thebypass passage 8 as shown by solid arrows in FIG. 15. The pressure ofthe refrigerant is increased in the first compressor 1 and therefrigerant is discharged therefrom, thereby causing an increase in thepressure at the suction port of the expander 3. As a result of this, apressure difference is produced between before and after the expander 3,as shown in FIG. 16, so that the expander 3 and the second compressor 5can be activated rapidly. After the expander 3 and the second compressor5 are activated, the bypass valve 9 is closed. The refrigerant that hasflowed out of the evaporator 4 is drawn into the second compressor 5through the flow passage 10 d as shown by dashed arrows in FIG. 15. Inthis way, smooth transfer to regular operation can be achieved byproviding the bypass passage 8.

CITATION LIST Patent Literature

Patent Literature 1: JP 2003-307358 A

SUMMARY OF INVENTION Technical Problem

In the refrigeration cycle apparatus 700, only the expander 3 isinvolved in the activation of the expander 3 and the second compressor5, whereas the second compressor 5 does not contribute thereto. Rather,the second compressor 5 is a load during the activation of the expander3. That is, friction, etc., between the power-recovery shaft 7 and thecomponents of the second compressor 5 cause a driving resistance to theexpander 3.

Meanwhile, in the regular operation of the refrigeration cycle apparatus700, the second compressor 5 and the expander 3 form the refrigerantcircuit 6 of a single channel, and their rotational speeds are identicalbecause they are coupled to each other by the power-recovery shaft 7that is commonly shared. Accordingly, the volume of the secondcompressor 5 and the volume of the expander 3 need to be set so that themass of the refrigerant to be drawn by the second compressor 5 per unittime should be equal to the mass of the refrigerant to be drawn by theexpander 3 per unit time.

FIG. 17 is an example of the Mollier diagram when carbon dioxide is usedas the refrigerant in the conventional refrigeration cycle apparatus700. As shown in FIG. 17, in the regular operation of the conventionalrefrigeration cycle apparatus 700, the refrigerant drawn by the secondcompressor 5 has a pressure of 40 kg/cm², and a temperature of about 10°C. (point A in FIG. 17), and the refrigerant has a density of 108.0kg/m³. The refrigerant drawn by the expander 3 has a pressure of 100kg/cm², and a temperature of 40° C. (point C in FIG. 17), and therefrigerant has a density of 628.61 kg/m³ at this time point.

Here, the suction volume (m³) of the second compressor 5 is referred toas Vc, the suction volume (m³) of the expander 3 is referred to as Ve,and the rotational speed (S⁻¹) of the power-recovery shaft 7 per secondis referred to as N. The mass (kg/s) of the refrigerant that the secondcompressor 5 can draw per second and the mass (kg/s) of the refrigerantthat the expander 3 can draw per second can be expressed respectively byFormula 1 and Formula 2.

(The mass of the refrigerant that the second compressor 5 can draw persecond)=108.0×Vc×N   (Formula 1)

(The mass of the refrigerant that the expander 3 can draw persecond)=628.61×Ve×N   (Formula 2)

When the mass of the refrigerant that the second compressor 5 can drawper second and the mass of the refrigerant that the expander 3 can drawper second are equal, the suction volume Vc of the second compressor 5can be expressed by Formula 3 from the above-mentioned Formula 1 andFormula 2.

Vc=(628.61/108.0)×Ve≈5.8×Ve   (Formula 3)

That is, in the activation of the refrigeration cycle apparatus 700, theexpander 3 is required to drive the second compressor 5 having a suctionvolume that is about 5.8 times that of the expander 3. Further, thelarger the ratio between the density of the refrigerant to be drawn bythe second compressor 5 and the density of the refrigerant to be drawnby the expander 3, the larger the ratio between the suction volume ofthe second compressor 5 and the suction volume of the expander 3 alsoshould be. In other words, the suction volume of the expander 3 becomessmaller with respect to the suction volume of the second compressor 5,and the driving resistance to the expander 3 in the activation of thesecond compressor 5 becomes relatively larger. Accordingly, there is apossibility that the expander 3 cannot drive the second compressor 5 atthe time of activation, depending on the operational conditions of therefrigeration cycle apparatus 700. Instead, it might be necessary toimpose an excess pressure, as compared to that in the regular operation,on the suction port side of the expander 3 in order to obtain a drivingforce necessary to drive the second compressor 5. As a result, the inputpower to the first compressor 1 increases and the efficiency of therefrigeration cycle apparatus 700 is reduced.

The present invention aims to solve the above-mentioned problems, and itis an object of the present invention to provide a refrigeration cycleapparatus that can be activated surely and stably.

Solution to Problem

That is, the present invention provides a refrigeration cycle apparatusincluding: a main refrigerant circuit having a low-pressure stagecompressor that compresses a refrigerant, a high-pressure stagecompressor that further compresses the refrigerant that has beencompressed in the low-pressure stage compressor, a heat radiator thatcools the refrigerant that has been compressed in the high-pressurestage compressor, an expander that recovers power from the refrigerantthat has been cooled in the heat radiator while expanding therefrigerant, the expander being coupled to the low-pressure stagecompressor by a shaft so that the recovered power is transferred to thelow-pressure stage compressor, a gas-liquid separator that separates therefrigerant that has been expanded in the expander into a gasrefrigerant and a liquid refrigerant, and an evaporator that evaporatesthe liquid refrigerant that has been separated in the gas-liquidseparator; an injection flow passage that introduces the gas refrigerantthat has been separated in the gas-liquid separator into a portion ofthe main refrigerant circuit from the discharge port of the low-pressurestage compressor to the suction port of the high-pressure stagecompressor; a high-pressure supply passage that communicates a portionof the main refrigerant circuit from the discharge port of thehigh-pressure stage compressor to the suction port of the expander and aportion of the main refrigerant circuit from the outlet of theevaporator to the suction port of the low-pressure stage compressor; anda flow passage-switching mechanism capable of selectively connecting oneselected from the evaporator and the high-pressure supply passage to thelow-pressure stage compressor.

From another aspect, the present invention provides a refrigerationcycle apparatus including: a main refrigerant circuit having alow-pressure stage compressor that compresses a refrigerant, ahigh-pressure stage compressor that further compresses the refrigerantthat has been compressed in the low-pressure stage compressor, a heatradiator that cools the refrigerant that has been compressed in thehigh-pressure stage compressor, an expander that recovers power from therefrigerant that has been cooled in the heat radiator while expandingthe refrigerant, the expander being coupled to the low-pressure stagecompressor by a shaft so that the recovered power is transferred to thelow-pressure stage compressor, a gas-liquid separator that separates therefrigerant that has been expanded in the expander into a gasrefrigerant and a liquid refrigerant, an evaporator that evaporates theliquid refrigerant that has been separated in the gas-liquid separator,and an expansion valve provided on the flow passage between thegas-liquid separator and the evaporator; an injection flow passage thatintroduces the gas refrigerant that has been separated in the gas-liquidseparator into a portion of the main refrigerant circuit from thedischarge port of the low-pressure stage compressor to the suction portof the high-pressure stage compressor; a controller that fully closesthe expansion valve in the activation of the refrigeration cycleapparatus so that the pressure at the suction port of the low-pressurestage compressor is prevented from being equal to the pressure at thedischarge port of the low-pressure stage compressor via the injectionflow passage.

Advantageous Effects of Invention

According to the refrigeration cycle apparatus of the present invention,the high-pressure stage compressor can draw the refrigerant in theevaporator and the gas-liquid separator through the injection flowpassage. This allows the pressure on the high-pressure side of the mainrefrigerant circuit to increase rapidly.

Since a portion of the main refrigerant circuit from the discharge portof the low-pressure stage compressor to the suction port of thehigh-pressure stage compressor is connected to the gas-liquid separatorby the injection flow passage, the pressure at the discharge port of theexpander can be made equal to the pressure at the suction port of thehigh-pressure stage compressor. The pressure at the suction port of theexpander is normally equal to the pressure on the high-pressure side ofthe main refrigerant circuit.

On the other hand, the pressure at the suction port of the low-pressurestage compressor can be made equal to the pressure on the high-pressureside of the main refrigerant circuit due to the functions of the flowpassage-switching mechanism and the high-pressure supply passage. Thepressure at the discharge port of the low-pressure stage compressor isnormally equal to the pressure at the suction port of the high-pressurestage compressor.

In this way, according to the present invention, a pressure differencecan be produced not only before and after the expander but also beforeand after the low-pressure stage compressor. Therefore, therefrigeration cycle apparatus of the present invention can be activatedsurely and stably, independent of operational conditions.

According to another aspect of the refrigeration cycle apparatus of thepresent invention, the high-pressure stage compressor can draw therefrigerant in the gas-liquid separator through the injection flowpassage. This allows the pressure on the high-pressure side of the mainrefrigerant circuit to increase rapidly.

Since a portion of the main refrigerant circuit from the discharge portof the low-pressure stage compressor to the suction port of thehigh-pressure stage compressor is connected to the gas-liquid separatorby the injection flow passage, the pressure at the discharge port of theexpander can be made equal to the pressure at the suction port of thehigh-pressure stage compressor. The pressure at the suction port of theexpander is normally equal to the pressure on the high-pressure side ofthe main refrigerant circuit.

On the other hand, the flow passages before and after the expansionvalve can be separated from each other by fully closing the expansionvalve. Accordingly, the pressure at the suction port of the low-pressurestage compressor can be prevented from being equal to the pressure atthe discharge port of the low-pressure stage compressor via theinjection flow passage. As a result, the pressure at the suction port ofthe low-pressure stage compressor can be maintained at the pressure ofthe main refrigerant circuit before the high-pressure stage compressoris driven (intermediate pressure). The pressure at the discharge port ofthe low-pressure stage compressor is normally equal to the pressure atthe suction port of the high-pressure stage compressor.

In this way, according to the present invention, a pressure differencecan be produced not only before and after the expander but also beforeand after the low-pressure stage compressor. Therefore, therefrigeration cycle apparatus of the present invention can be activatedsurely and stably, independent of operational conditions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram of a refrigeration cycle apparatus inEmbodiment 1 of the present invention.

FIG. 2 is a flow chart of the activation control of the refrigerationcycle apparatus in Embodiment 1 of the present invention.

FIG. 3 is a configuration diagram of a refrigeration cycle apparatus inModification 1.

FIG. 4 is a flow chart of the activation control of the refrigerationcycle apparatus in Modification 1.

FIG. 5 is a configuration diagram of a refrigeration cycle apparatus inModification 2.

FIG. 6 is a schematic view showing the state in the activation of therefrigeration cycle apparatus in Embodiment 1, Modification 1, andModification 2.

FIG. 7 is a configuration diagram of a power recovery system.

FIG. 8 is a configuration diagram of a refrigeration cycle apparatus inEmbodiment 2 of the present invention.

FIG. 9 is a flow chart of the activation control of the refrigerationcycle apparatus in Embodiment 2 of the present invention.

FIG. 10 is a configuration diagram of a refrigeration cycle apparatus inModification 3.

FIG. 11 is a flow chart of the activation control of the refrigerationcycle apparatus in Modification 3.

FIG. 12 is a configuration diagram of a refrigeration cycle apparatus inModification 4.

FIG. 13 is a flow chart of the activation control of the refrigerationcycle apparatus in Modification 4.

FIG. 14A is a schematic view showing the state in the activation of therefrigeration cycle apparatus in Embodiment 2 and Modification 3.

FIG. 14B is a schematic view showing the state in the activation of therefrigeration cycle apparatus in Modification 4.

FIG. 15 is a configuration diagram of a conventional refrigeration cycleapparatus.

FIG. 16 is a schematic view showing the state in the activation of therefrigeration cycle apparatus shown in FIG. 15.

FIG. 17 is a Mollier diagram when carbon dioxide is used as arefrigerant in the conventional refrigeration cycle apparatus.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention are described withreference to drawings. It should be noted that the present invention isnot limited to the following embodiments.

Embodiment 1

<Configuration of Refrigeration Cycle Apparatus 100

FIG. 1 is a configuration diagram showing a refrigeration cycleapparatus 100 in Embodiment 1 of the present invention. As shown in FIG.1, the refrigeration cycle apparatus 100 is provided with a refrigerantcircuit 106 formed by sequentially connecting a high-pressure stagecompressor 101, a heat radiator 102, an expander 103, a gas-liquidseparator 108, an evaporator 104, and a low-pressure stage compressor105, with flow passages 106 a to 106 f. The flow passages 106 a to 106 feach are constituted by a refrigerant pipe. An expansion valve 110 isprovided on the flow passage 106 d between the gas-liquid separator 108and the evaporator 104. A check valve 132 is provided on the flowpassage 106 e between the evaporator 104 and the low-pressure stagecompressor 105. Hereinafter, the flow passage 106 f connecting thedischarge port of the low-pressure stage compressor 105 and the suctionport of the high-pressure stage compressor 101 may be referred to alsoas the “intermediate-pressure flow passage 106 f”.

The high-pressure stage compressor 101 is constituted by a compressionmechanism 101 a and a motor 101 b for driving the compression mechanism101 a. The high-pressure stage compressor 101 compresses the refrigerantto high temperature and high pressure. A positive displacementcompressor such as a scroll compressor and a rotary compressor can beused as the high-pressure stage compressor 101. The discharge port ofthe high-pressure stage compressor 101 is connected to the inlet of theheat radiator 102 via the flow passage 106 a.

The heat radiator 102 radiates heat (cools) of the refrigerant at hightemperature and high pressure that has been compressed by thehigh-pressure stage compressor 101 through heat exchange with anexternal heat source. The outlet of the heat radiator 102 is connectedto the suction port of the expander 103 via the flow passage 106 b.

The expander 103 expands the refrigerant at intermediate temperature andhigh pressure that has flowed out of the heat radiator 102 and convertsthe expansion energy (power) of the refrigerant into mechanical energyto recover it. The discharge port of the expander 103 is connected tothe inlet of the gas-liquid separator 108 via the flow passage 106 c. Apositive displacement expander such as a scroll expander and a rotaryexpander can be used as the expander 103. In addition, a fluid pressuremotor expander can be also used as the expander 103. The fluid pressuremotor expander is a positive displacement fluid machine that recoverspower from a refrigerant by sequentially performing processes of drawingthe refrigerant and discharging the drawn refrigerant without performingany substantial expansion process in a working chamber. The detailedstructure and the operational principle of the fluid pressure motorexpander are disclosed, for example, in WO 2008/050654 A.

The gas-liquid separator 108 serves to separate the refrigerant at lowtemperature and low pressure that has been expanded in the expander 103into a gas refrigerant and a liquid refrigerant. The gas-liquidseparator 108 can prevent the liquid refrigerant from being drawn intothe high-pressure stage compressor 101 in a large amount in theactivation of the refrigeration cycle apparatus 100. The gas refrigerantoutlet of the gas-liquid separator 108 is connected to the flow passage106 f via an injection flow passage 111. The liquid-refrigerant outletof the gas-liquid separator 108 is connected to the inlet of theevaporator 104 via the flow passage 106 d provided with the expansionvalve 110.

The expansion valve 110 serves to regulate the flow rate of the liquidrefrigerant to flow into the evaporator 104 in the regular operation.Accordingly, a valve, which allows the degree of opening to be variedstepwise, capable of expanding a refrigerant, typically, an electricexpansion valve is preferably used as the expansion valve 110. In theactivation of the refrigeration cycle apparatus 100, the expansion valve110 is fully opened or substantially fully opened. This allows therefrigerant in the evaporator 104 to be drawn by the high-pressure stagecompressor 101 smoothly.

The evaporator 104 evaporates the liquid refrigerant at low temperatureand low pressure that has been separated in the gas-liquid separator 108through heat exchange with an external heat source. The outlet of theevaporator 104 is connected to the suction port of the low-pressurestage compressor 105 via the flow passage 106 e provided with the checkvalve 132.

The low-pressure stage compressor 105 draws the refrigerant atintermediate temperature and low pressure that has flowed out of theevaporator 104, and discharges it into the intermediate-pressure flowpassage 106 f after preliminarily increasing the pressure thereof. Thedischarge port of the low-pressure stage compressor 105 is connected tothe suction port of the high-pressure stage compressor 101 via theintermediate-pressure flow passage 106 f. A positive displacementcompressor such as a scroll compressor and a rotary compressor can beused as the low-pressure stage compressor 105. Further, a fluid pressuremotor compressor can be used as the low-pressure stage compressor 105.The fluid pressure motor compressor is a positive displacement fluidmachine that increases the pressure of a refrigerant by substantiallysequentially performing processes of drawing the refrigerant from theevaporator 104 and discharging the refrigerant to the high-pressurestage compressor 101. In other words, the fluid pressure motorcompressor is a fluid machine that allows substantially no change in thevolume of the refrigerant in a working chamber. The fluid pressure motorcompressor basically has the same structure as the fluid pressure motorexpander, the detail of which is disclosed in the above-mentionedliterature.

The expander 103 is coupled to the low-pressure stage compressor 105 bya power-recovery shaft 107. The mechanical energy (power) recovered inthe expander 103 can be transferred to the low-pressure stage compressor105 via the power-recovery shaft 107. That is, the expander 103, thelow-pressure stage compressor 105, and the power-recovery shaft 107function as a power recovery system 109 that recovers power from therefrigerant. As shown in FIG. 7, the expander 103 and the low-pressurestage compressor 105 are accommodated in a single closed casing 109 aholding lubrication oil, together with the power-recovery shaft 107.Therefore, no particular sealing structure is needed.

In this embodiment, the low-pressure stage compressor 105 has a largervolume than the expander 103. In the case of using carbon dioxide as therefrigerant, the ratio (Vc/Ve) of the volume Vc of the low-pressurestage compressor 105 with respect to the volume Ve of the expander 103is set, for example, to the range of 5 to 15. In the case of using R410Aas the refrigerant, the ratio (Vc/Ve) is set, for example, to the rangeof 30 to 40. In the case of using a thin refrigerant like R410A as therefrigerant to be drawn into the compressor, the ratio (Vc/Ve) alsotends to increase. Generally, the larger the ratio (Vc/Ve), the largerthe driving force (torque) required for self-activation of thelow-pressure stage compressor 105 and the expander 103 should be. Inthis regard, “the volume of the low-pressure stage compressor 105” meansa confined volume, that is, the volume of the working chamber at thetime of completion of the drawing process. This applies also to theexpander 103.

The refrigeration cycle apparatus 100 is further provided with ahigh-pressure supply passage 130 and an on-off valve 131. Thehigh-pressure supply passage 130 is connected to the main refrigerantcircuit 106 so as to communicate the flow passage 106 a and the flowpassage 106 e. The on-off valve 131 is provided on the high-pressuresupply passage 130 and controls the flow of the refrigerant in thehigh-pressure supply passage 130.

The high-pressure supply passage 130 has an upstream end E₁ (one end)connected to the flow passage 106 a and a downstream end E₂ (the otherend) connected to the flow passage 106 e. That is, the high-pressuresupply passage 130 is a flow passage that can introduce the refrigerantin the flow passage 106 a directly to the suction port of thelow-pressure stage compressor 105 before the rotation of thepower-recovery shaft 107. The high-pressure supply passage 130,typically, is constituted by a refrigerant pipe.

As long as the pressure at the suction port of the low-pressure stagecompressor 105 can be increased in the activation of the refrigerationcycle apparatus 100, the position of the upstream end E₁ is not limitedto the position shown in FIG. 1. That is, the position of the upstreamend E₁ is not specifically limited, as long as a portion of the mainrefrigerant circuit 106 from the discharge port of the high-pressurestage compressor 101 to the suction port of the expander 103 and aportion of the main refrigerant circuit 106 from the outlet of theevaporator 104 to the suction port of the low-pressure stage compressor105 can be communicated. Specifically, the high-pressure supply passage130 may be connected to the main refrigerant circuit 106 so as tocommunicate the flow passage 106 b and the flow passage 106 e. Dependingon the circumstances, the high-pressure supply passage 130 may branchfrom the heat radiator 102. For example, in the case where the heatradiator 102 is constituted by an upstream part and a downstream part,the high-pressure supply passage 130 can easily branch from a portionbetween these two parts.

However, in the case where the upstream end E₁ is located on the flowpassage 106 a as shown in FIG. 1, the following effects are obtained.The density of the refrigerant in the flow passage 106 a is lower thanthe density of the refrigerant in the flow passage 106 b. Normally, therefrigerant in the flow passage 106 a is in a gas phase. It is possibleto prevent the liquid refrigerant from flowing into the high-pressurestage compressor 101 through the low-pressure stage compressor 105 bysupplying the gas refrigerant to the suction port of the low-pressurestage compressor 105 through the high-pressure supply passage 130. Thisprevents liquid compression in the high-pressure stage compressor 101,leading to an enhancement in reliability of the refrigeration cycleapparatus 100.

The on-off valve 131 and the check valve 132 form a flowpassage-switching mechanism capable of selectively connecting oneselected from the evaporator 104 and the high-pressure supply passage130 to the low-pressure stage compressor 105. When one selected from theevaporator 104 and the high-pressure supply passage 130 is selectivelyconnected to the low-pressure stage compressor 105, the refrigerant isintroduced from one selected from the evaporator 104 and thehigh-pressure supply passage 130 to the low-pressure stage compressor105. The check valve 132 is provided in a portion of the mainrefrigerant circuit 106 from the outlet of the evaporator 104 to thedownstream end E₂ of the high-pressure supply passage 130 (flow passage106 e).

The on-off valve 131 is closed in the regular operation, and is openedin the activation of the refrigeration cycle apparatus 100. By openingthe on-off valve 131, it is possible to supply the refrigerant in theflow passage 106 a directly to the suction port of the low-pressurestage compressor 105 through the high-pressure supply passage 130. Atthat time, the check valve 132 can block the flow of the refrigerantfrom the high-pressure supply passage 130 toward the evaporator 104. Onthe other hand, by closing the on-off valve 131, it is possible tosupply the refrigerant from the evaporator 104 to the low-pressure stagecompressor 105 while restricting the flow of the refrigerant from thehigh-pressure supply passage 130 to the low-pressure stage compressor105. The check valve 132 has an advantage that there is no need forelectrical control. Of course, it is also possible to replace the checkvalve 132 with a valve that can be arbitrarily opened and closed.

The refrigeration cycle apparatus 100 is further provided with theinjection flow passage 111 and an injection flow-regulating valve 112.The injection flow passage 111 serves to introduce the gas refrigerantseparated from the liquid refrigerant in the gas-liquid separator 108,to a portion of the main refrigerant circuit 106 from the discharge portof the low-pressure stage compressor 105 to the suction port of thehigh-pressure stage compressor 101 (intermediate-pressure flow passage106 f). Specifically, the injection flow passage 111 is connected to themain refrigerant circuit 106 so as to communicate the gas refrigerantoutlet of the gas-liquid separator 108 and the intermediate-pressureflow passage 106 f. The injection flow-regulating valve 112 is providedon the injection flow passage 111 and controls the flow of therefrigerant in the injection flow passage 111. The injection flowpassage 111, typically, is constituted by a refrigerant pipe. As theinjection flow-regulating valve 112, a valve, which allows the degree ofopening to be varied stepwise, capable of expanding a refrigerant,typically, an electric expansion valve is preferably used.

The injection flow-regulating valve 112 serves to regulate the flow rateof the gas refrigerant to be injected into the intermediate-pressureflow passage 106 f in the regular operation. On the other hand, theinjection flow-regulating valve 112 is fully opened or substantiallyfully opened in the activation of the refrigeration cycle apparatus 100.When the injection flow-regulating valve 112 is opened in theactivation, the high-pressure stage compressor 101 can draw therefrigerant present in the flow passage 106 c, the gas-liquid separator108, the flow passage 106 d, and the evaporator 104. This enables thepressure on the high-pressure side of the main refrigerant circuit 106to increase rapidly. Particularly, since the gas-liquid separator 108 isprovided in this embodiment, it is possible to store a sufficient amountof refrigerant between the discharge port of the expander 103 and thecheck valve 132 during stoppage.

In order to improve the coefficient of performance of the refrigerationcycle apparatus 100, the gas refrigerant can be supplied from thegas-liquid separator 108 to the intermediate-pressure flow passage 106 fthrough the injection flow passage 111 after the activation of therefrigeration cycle apparatus 100. By appropriately adjusting the degreeof opening of each of the expansion valve 110 and the injectionflow-regulating valve 112, it is possible to prevent the liquidrefrigerant from flowing from the gas-liquid separator 108 into theintermediate-pressure flow passage 106 f as well as preventing thebackflow of the refrigerant from the intermediate-pressure flow passage106 f to the gas-liquid separator 108.

The refrigeration cycle apparatus 100 is further provided with acontroller 117. The expansion valve 110, the injection flow-regulatingvalve 112, and the on-off valve 131 are controlled by the controller117. The controller 117, typically, is constituted by a microcomputer.When a command to start the operation of the refrigeration cycleapparatus 100 is given to the controller 117 through an input device(not shown), a predetermined control program stored in an internalmemory of the controller 117 is executed. Specifically, the controller117 executes the predetermined activation control described below withreference to FIG. 2. Further, the controller 117 controls the action ofthe motor 101 b that drives the high-pressure stage compressor 101.

The refrigeration cycle apparatus 100 is further provided with anactivation detector 119 that detects the activation of the expander 103or the low-pressure stage compressor 105. The controller 117 switchesthe control of the on-off valve 131 (flow passage-switching mechanism)from the control before the activation to the control after theactivation, according to a detection result of the activation detector119. Specifically, the on-off valve 131 is opened before the activationof the expander 103 and the low-pressure stage compressor 105 so thatthe refrigerant is introduced from the high-pressure supply passage 130to the low-pressure stage compressor 105. After the activation of theexpander 103 and the low-pressure stage compressor 105, the on-off valve131 is closed so that the refrigerant is introduced from the evaporator104 to the low-pressure stage compressor 105. For example, upon thereception of signals that indicate the activation of the low-pressurestage compressor 105 from the activation detector 119, the controller117 closes the on-off valve 131. In this way, smooth transfer to thecontrol in the regular operation can be achieved.

A temperature detector, a pressure detector, or the like can be used asthe activation detector 119. The activation detector 119 as atemperature detector, for example, includes a temperature detectingelement such as a thermocouple and a thermistor, and detects thedifference ΔT between the temperature of the refrigerant at the suctionport of the expander 103 and the temperature of the refrigerant at thedischarge port of the expander 103. The activation detector 119 as apressure detector, for example, includes a piezoelectric element, anddetects the difference ΔP between the pressure of the refrigerant at thesuction port of the expander 103 and the pressure of the refrigerant atthe discharge port of the expander 103. Further, the activation detector119 may include a timer that measures an elapsed time from theactivation of the high-pressure stage compressor 101. Such a timer canbe provided also as a function of the controller 117. In this case, thecontroller 117 itself can function as the activation detector 119.Furthermore, a contact or noncontact displacement sensor that detectsthe driving of the power-recovery shaft 107, such as an encoder, may beprovided as the activation detector 119.

Depending on the type of the activation detector 119, the method fordetecting the activation of the power recovery system 109 differs asfollows. According to the following methods, it is possible to detectthe activation of the power recovery system 109 easily.

In the case of using a pressure detector as the activation detector 119,a threshold P_(th) that has been determined experimentally ortheoretically, for example, is preset in the controller 117. When thevalue obtained by subtracting the current pressure difference ΔP_(n+1)detected by the pressure detector from the pressure difference ΔP_(n)(n: natural number) that has been detected by the pressure detector at atime going back for a unit time exceeds the specific threshold P_(th),the activation of the expander 103 or the low-pressure stage compressor105 is detected. In the controller 117, a single threshold P_(th) may beset, or a plurality of thresholds P_(th) associated with the outdoortemperature or the like may be set. In the latter case, the controller117 selects an optimal threshold P_(th) on the basis of the outdoortemperature or the like. This applies also to other thresholds describedbelow.

The difference ΔP between the pressure of the refrigerant at the suctionport of the expander 103 and the pressure of the refrigerant at thedischarge port of the expander 103 generally monotonically increasesduring the period after the activation of the high-pressure stagecompressor 101 and before the activation of the expander 103. When theexpander 103 starts to operate, the pressure difference ΔP turns todecrease temporarily, and becomes smaller than that immediately beforethe activation of the expander 103. It is possible to detect theactivation of the expander 103 or the low-pressure stage compressor 105by capturing this change in the pressure difference ΔP. Specifically,the pressure difference ΔP is detected at every unit time and stored inthe memory of the controller 117. Then, the last pressure differenceΔP_(n) previously stored in the memory and the current pressuredifference ΔP_(n+1) are compared. When the current pressure differenceΔP₊ 1 significantly falls below the last past pressure differenceΔP_(n), the expander 103 or the low-pressure stage compressor 105 can bedetermined to have been activated. In other words, when(ΔP_(n)−ΔP_(n+1))>P_(th) is satisfied, the expander 103 or thelow-pressure stage compressor 105 can be determined to have beenactivated. It should be noted that the “unit time” can be setarbitrarily to a sufficient time to capture a sudden decrease in thepressure difference ΔP, for example, in the range of 1 to 5 seconds.

Instead of the pressure difference ΔP, it is also possible to use thetemperature difference ΔT. That is, when a value obtained by subtractingthe current temperature difference ΔT_(n+1) detected by the temperaturedetector from the temperature difference ΔT_(n) (n: natural number) thathas been detected by the temperature detector at a time going back for aunit time exceeds a specific threshold T_(th), the activation of theexpander 103 or the low-pressure stage compressor 105 is detected.

Further, there is also a possibility that the activation of the powerrecovery system 109 can be detected on the basis of the dischargetemperature from the expander 103 or the discharge pressure from theexpander 103. When the power recovery system 109 is activated, theexpander 103 is also rotated. The expander 103 draws the refrigerant,then expands the drawn refrigerant, and discharges it. Therefore, thetemperature and pressure of the refrigerant after being discharged fromthe expander 103 are lower than those before being drawn. The powerrecovery system 109 can be determined to have been activated bycapturing a sudden change in the temperature (or pressure) at thedischarge port of the expander 103 while monitoring the temperature (orpressure) in chronological order.

Meanwhile, on the presumption that the power recovery system 109 isactivated without fail, the activation of the expander 103 or thelow-pressure stage compressor 105 may be detected by the methoddescribed below. The below-described method is rather a method ofdetermining whether the power recovery system 109 is in a state thatallows continuous operation, than a method of capturing the activationof the expander 103 or the low-pressure stage compressor 105. After theactivation of the expander 103 or the low-pressure stage compressor 105is detected by the below-described method, the control of the on-offvalve 131 (flow passage-switching mechanism) can be switched from thecontrol before the activation to the control after the activationaccording to the detected results. In this way, the power recoverysystem 109 continues to operate stably even after the on-off valve 131is closed.

Specifically, in the case of using a temperature detector as theactivation detector 119, a threshold T₁ that has been determinedexperimentally or theoretically, for example, is preset in thecontroller 117. When the temperature difference ΔT detected by thetemperature detector exceeds the threshold T₁, the activation of theexpander 103 or the low-pressure stage compressor 105 is detected.

In the case of using a pressure detector as the activation detector 119,a threshold P₁ that has been determined experimentally or theoretically,for example, is preset in the controller 117. When the pressuredifference ΔP detected by the pressure detector exceeds the specificthreshold P₁, the activation of the expander 103 or the low-pressurestage compressor 105 is detected.

The reason why the activation of the expander 103 or the low-pressurestage compressor 105 can be detected by comparison between thetemperature difference ΔT and the threshold T₁, or comparison betweenthe pressure difference ΔP and the threshold P₁ is as follows. When thehigh-pressure stage compressor 101 is activated, the refrigerantdischarged from the high-pressure stage compressor 101 is supplied tothe suction port of the low-pressure stage compressor 105 through thehigh-pressure supply passage 130. This activates the power recoverysystem 109. At this time, since the low-pressure stage compressor 105serves as a driving source, the power recovery system 109 starts torotate before the temperature difference between the suction temperatureof the high-pressure stage compressor 101 and the discharge temperatureof the high-pressure stage compressor 101 becomes significant. When thepower recovery system 109 starts to rotate, the pressure difference inthe cycle has not been sufficiently increased, and the power to rotatethe power recovery system 109 is low. Therefore, the rotational speed ofthe power recovery system 109 is also low. When the rotational speed ofthe power recovery system 109 is low, the rotational speed of theexpander 103 is also low. This state corresponds to a “narrowed state”that is a description for expansion valves. Accordingly, the dischargetemperature and discharge pressure of the high-pressure stage compressor101 gradually increase as well.

When the discharge temperature and discharge pressure of thehigh-pressure stage compressor 101 increase, the power to rotate theexpander 103 and the low-pressure stage compressor 105 also increases,and the rotational speed of the power recovery system 109 increases.Then, when a high rotational speed is achieved, the power recoverysystem 109 stably rotates under the influence of the inertial force. Theon-off valve 131 is desirably maintained open until such a stablerotation state is reached.

On the other hand, the suction temperature of the expander 103 graduallyincreases from the temperature during stoppage, which is substantiallythe same as the outdoor temperature. Depending on the suctiontemperature (or suction pressure) of the expander 103, the dischargetemperature (or discharge pressure) of the expander 103 is determined.The suction temperature, the discharge temperature, the suctionpressure, and the discharge pressure of the expander 103 each in theactivation of the power recovery system 109 and in the regular operationof the power recovery system 109, for example, with the outdoortemperature being 10° C., are shown below as an example. It should benoted that the following values are calculated with the expansionratio=2.0. The refrigerant is carbon dioxide.

<In Activation>

Suction temperature: 10° C.

Suction pressure: 5.0 MPa

Discharge temperature: −3.0° C.

Discharge pressure: 3.2 MPa

Difference between suction temperature and discharge temperature: 13° C.

Difference between suction pressure and discharge pressure: 1.8 MPa

<In Regular Operation>

Suction temperature: 40° C.

Suction pressure: 10.0 MPa

Discharge temperature: 13.4° C.

Discharge pressure: 4.9 MPa

Difference between suction temperature and discharge temperature: 26.6°C. Difference between suction pressure and discharge pressure: 5.1 MPa

When the power recovery system 109 is activated with the dischargetemperature and discharge pressure of the high-pressure stage compressor101 being low, the suction temperature of the expander 103 and thedischarge temperature of the expander 103 each gradually increase asmentioned above. The difference between the suction temperature and thedischarge temperature gradually increase as well. This applies also tothe pressure. Therefore, the activation of the power recovery system 109can be detected by setting appropriate values as the threshold T₁ andthe threshold P₁, e.g., respective values that are slightly larger thanthe temperature difference and the pressure difference to be reachedwhen the activation of the power recovery system 109 can be estimated.

In the case of using a timer as the activation detector 119, a thresholdtime t₁ that has been determined experimentally or theoretically, forexample, is preset in the controller 117. When the time t measured bythe timer exceeds the threshold time t₁, the activation of the expander103 or the low-pressure stage compressor 105 is detected.

The “threshold time t₁” is written in the activation control program tobe executed by the controller 117. For example, the time from theactivation of the high-pressure stage compressor 101 to the activationof the low-pressure stage compressor 105 is actually measured undervarious operational conditions (such as outdoor temperatures). Then, atime at which the activation of the low-pressure stage compressor 105can be surely determined under all the operational conditions can be setas the “threshold time t₁”. Theoretically, a model of the refrigerationcycle apparatus 100 is constructed, and a pressure difference that isnecessary and sufficient to activate the power recovery system 109 isestimated by computer simulation. Thereafter, using parameters such asthe volume of the high-pressure stage compressor 101 and the fillingamount of the refrigerant in the main refrigerant circuit 106, theinitial activation time necessary to produce the estimated pressuredifference is calculated. The calculated initial activation time can beset as the “threshold time t₁”.

The method for detecting the activation of the expander 103 or thelow-pressure stage compressor 105 is not limited to one, and a pluralityof methods can be performed in combination. For example, the activationof the expander 103 or the low-pressure stage compressor 105 isaccurately captured by a method of monitoring the pressure difference ΔPand/or the temperature difference ΔT between the suction port and thedischarge port of the expander 103. Thereafter, whether the powerrecovery system 109 is in a state that allows continuous operation isdetermined by a method of comparing the temperature difference ΔT withthe threshold T₁, a method of comparing the pressure difference ΔP withthe threshold P₁, or a method of comparing the elapsed time t with thethreshold time t₁. When these plurality of conditions are satisfied, theexpander 103 or the low-pressure stage compressor 105 is determined tobe activated, and the on-off valve 131 is closed.

<Operation of Refrigeration Cycle Apparatus 100>

FIG. 2 is a flow chart showing the activation control of therefrigeration cycle apparatus 100. The refrigeration cycle apparatus 100starts the regular operation after performing the activation controlshown in FIG. 2. In an operation standby state, the high-pressure stagecompressor 101 is stopped, the expansion valve 110 is opened, and thepressure of the refrigerant in the main refrigerant circuit 106 issubstantially uniform.

When an activation command is input in step S11, the controller 117transmits control signals to the actuators of the expansion valve 110and the injection flow-regulating valve 112 so that these valves 110 and112 are fully opened (step S12). Further, it transmits control signalsto the actuator of the on-off valve 131 so that the on-off valve 131 isopened (step S13). This allows the high-pressure supply passage 130 tobe open.

Next, the controller 117 starts to supply power to the motor 101 b inorder to activate the high-pressure stage compressor 101 (step S14).This activates the high-pressure stage compressor 101 and causes therefrigerant present in the intermediate-pressure flow passage 106 f, theinjection flow passage 111, the flow passage 106 c, the gas-liquidseparator 108, the flow passage 106 d, the evaporator 104, and a part ofthe flow passage 106 e (portion between the evaporator 104 and the checkvalve 132) to be drawn into the high-pressure stage compressor 101.

Instead of opening the on-off valve 131 before the activation of thehigh-pressure stage compressor 101, it is also possible to open theon-off valve 131 upon the activation of the high-pressure stagecompressor 101.

Upon the activation of the high-pressure stage compressor 101, a fan orpump that causes a fluid (air or water) for heat exchange with therefrigerant to flow into the heat radiator 102 is activated. This canprevent an excessive increase in the high pressure of the cycle.Likewise, a fan or pump of the evaporator 104 is activated upon theactivation of the high-pressure stage compressor 101. This allowsefficient production of the gas refrigerant to be drawn into thehigh-pressure stage compressor 101.

Once the drawing of the refrigerant into the high-pressure stagecompressor 101 is started, the internal pressure of theintermediate-pressure flow passage 106 f, etc., decreases. On the otherhand, since the refrigerant compressed in the high-pressure stagecompressor 101 is discharged, the pressure in flow passages from thedischarge port of the high-pressure stage compressor 101 to the suctionport of the expander 103 (the flow passage 106 a, the heat radiator 102,and the flow passage 106 b), the high-pressure supply passage 130, and apart of the flow passage 106 e (portion between the check valve 132 andthe suction port of the low-pressure stage compressor 105) increases.

As a result, as shown in FIG. 6, the pressure at the suction port ofeach of the expander 103 and the low-pressure stage compressor 105 isrendered relatively high, and the pressure at the discharge port of eachof the expander 103 and the low-pressure stage compressor 105 isrendered relatively low. That is, a pressure difference can be producednot only between the suction port and the discharge port of the expander103, but also between the suction port and the discharge port of thelow-pressure stage compressor 105. The pressure difference of therefrigerant acts on each of the expander 103 and the low-pressure stagecompressor 105, and thus self-activation of the power recovery system109 can be achieved easily. The high-pressure stage compressor 101 candraw a sufficient amount of the refrigerant to produce a large pressuredifference because the injection flow passage 111 and the gas-liquidseparator 108 are provided.

Upon detecting the activation of the low-pressure stage compressor 105through the activation detector 119 (step S15), the controller 117transmits control signals to the actuator of the on-off valve 131 sothat the on-off valve 131 is closed (step S16). This allows thebackpressure acting on the check valve 132 to be released, and therefrigerant is supplied from the evaporator 104 to the low-pressurestage compressor 105 through the flow passage 106 e. Meanwhile, thegas-liquid two-phase refrigerant whose pressure has been reduced in theexpander 103 is supplied to the gas-liquid separator 108. The openingdegree of each of the expansion valve 110 and the injectionflow-regulating valve 112 is adjusted so that excess supply of theliquid refrigerant to the high-pressure stage compressor 101 through theinjection flow passage 111 and the flow passage 106 f is prevented (stepS17). After the completion of the activation control shown in FIG. 2,transfer to the regular operation where the refrigerant is circulated inthe main refrigerant circuit 106 is performed in the refrigeration cycleapparatus 100.

When the operation of the refrigeration cycle apparatus 100 is intendedto be stopped, the rotational speed of the high-pressure stagecompressor 101, for example, is progressively reduced. After thehigh-pressure stage compressor 101 has been stopped, the refrigerantmoves through the high-pressure stage compressor 101, the expander 103,and the low-pressure stage compressor 105, taking sufficient time.Therefore, the pressure difference in the main refrigerant circuit 106is naturally released, and the pressure in the main refrigerant circuit106 becomes substantially uniform to be stabilized. This causes theexpander 103 and the low-pressure stage compressor 105 to be stoppednaturally.

<Effects of Refrigeration Cycle Apparatus 100>

According to this embodiment, the high-pressure stage compressor 101 candraw and compress the refrigerant in the evaporator 104 and thegas-liquid separator 108 in the activation of the refrigeration cycleapparatus 100. Therefore, the pressure in flow passages from thedischarge port of the high-pressure stage compressor 101 to the suctionport of the expander 103 can be rapidly increased. Since a largepressure difference is produced between the suction port and thedischarge port of the expander 103, the power recovery system 109 isself-activated smoothly.

Further, the compressed refrigerant is introduced into a part of theflow passage 106 e from the check valve 132 to the suction port of thelow-pressure stage compressor 105 through the high-pressure supplypassage 130. This produces a large pressure difference also between thesuction port and the discharge port of the low-pressure stage compressor105. This fact contributes to a smoother self-activation of the powerrecovery system 109. In this embodiment, the low-pressure stagecompressor 105 and the expander 103 each have a certain suction volume.Particularly, when the suction volume of the low-pressure stagecompressor 105 is larger than the suction volume of the expander 103,the power recovery system 109 is activated more smoothly by producing apressure difference between the suction port and the discharge port ofthe low-pressure stage compressor 105.

In the case where a certain period of time has elapsed without detectingthe activation of the expander 103 or the low-pressure stage compressor105 after the activation of the high-pressure stage compressor 101, itcan be determined that the activation of the power recovery system 109has been failed. In the case where the activation of the power recoverysystem 109 has been failed, the controller 117 stops the high-pressurestage compressor 101 and performs the control to activate the powerrecovery system 109 again. In this way, it is possible to prevent anexcessive increase in the pressure in flow passages from the dischargeport of the high-pressure stage compressor 101 to the suction port ofthe expander 103. It is also possible to prevent damages to thecomponents of the expander 103 from occurring due to an excessivepressure difference between before and after the expander 103. Thus, thereliability of the refrigeration cycle apparatus 100 can be improved.

According to this embodiment, the heat radiator 102 is connected to thesuction port of the expander 103, the evaporator 104 is connected to thesuction port of the low-pressure stage compressor 105, and thegas-liquid separator 108 is connected to the discharge port of theexpander 103. The gas-liquid separator 108 is connected also to thedischarge port of the low-pressure stage compressor 105 via theinjection flow passage 111. Since the volumes of the heat radiator 102,the evaporator 104, and the gas-liquid separator 108 are comparativelylarge, these components can function as a buffer space for therefrigerant in the activation of the refrigeration cycle apparatus 100.Thus, an effect of suppressing the pressure pulsation in the activationcan be obtained.

The type of refrigerant (working fluid) that can be used in thisembodiment is not specifically limited. For example, fluorinerefrigerant such as R410A, natural refrigerant such as carbon dioxide,and low GWP (Global Warming Potential) refrigerant such as R1234yf canbe used. Whichever the refrigerant is used, the above-mentioned effectscan be obtained.

Modification 1

<Configuration of Refrigeration Cycle Apparatus 200>

FIG. 3 is a configuration diagram of a refrigeration cycle apparatus 200in Modification 1. As shown in FIG. 3, the flow passage-switchingmechanism is constituted by a three-way valve 133 in the refrigerationcycle apparatus 200. As the activation detector 119, a PTC (PositiveTemperature Coefficient) heater 140 and a current detector 141 are used.Further, a bypass flow passage 201 and a bypass valve 202 are provided.Other configurations are the same as those in Embodiment 1. In thismodification, the same components as those in Embodiment 1 are denotedby the same reference numerals, and the detailed descriptions thereofare omitted.

The three-way valve 133 as a flow passage-switching mechanism isprovided at the downstream end E₂ of the high-pressure supply passage130 so as to be capable of switching between a first state in which therefrigerant is introduced from the evaporator 104 into the low-pressurestage compressor 105 and a second state in which the refrigerant isintroduced from the high-pressure supply passage 130 into thelow-pressure stage compressor 105. In the first state, the flow of therefrigerant from the high-pressure supply passage 130 to thelow-pressure stage compressor 105 is blocked. In the second state, theflow of the refrigerant from the evaporator 104 to the low-pressurestage compressor 105 is blocked. In this way, the on-off valve 131 andthe check valve 132 in Embodiment 1 can be replaced with the three-wayvalve 133. The three-way valve 133 can suppress an increase in thenumber of components.

The bypass flow passage 201 is connected to the main refrigerant circuit106 so as to bypass the expander 103. The upstream end E₃ of the bypassflow passage 201 is located on the flow passage 106 b, and thedownstream end E₄ thereof is located on the flow passage 106 c. Thebypass valve 202 is provided on the bypass flow passage 201. The bypassflow passage 201, typically, is constituted by a refrigerant pipe. Asthe bypass valve 202, a valve, which allows the degree of opening to bevaried stepwise, capable of expanding a refrigerant, typically, anelectric expansion valve is preferably used.

The current detector 141 detects the magnitude of the current flowing inthe PTC heater 140. The PTC heater 140 is provided in a portion of themain refrigerant circuit 106 from the outlet of the heat radiator 102 tothe suction port of the expander 103, that is, on the flow passage 106b. Specifically, the PTC heater 140 is located on the expander 103 sideas seen from the upstream end E₃ of the bypass flow passage 201. Whenthe PTC heater 140 is provided at such a position, the PTC heater 140 isless likely to be affected by the flow of the refrigerant toward thebypass flow passage 201. Therefore, it is possible to detect the flow ofthe refrigerant into the expander 103 accurately.

In the case of using the PTC heater 140 and the current detector 141 asthe activation detector 119, a threshold ΔI₁ that has been determinedexperimentally or theoretically, for example, is preset in thecontroller 117. When the power recovery system 109 is activated, therefrigerant starts to flow also at the suction port of the expander 103.Then, the magnitude of the current also suddenly changes due to thetemperature change (temperature reduction) of the PTC heater 140. Inorder to capture such a change, the amount of change per unit time inthe current flowing in the PTC heater 140 can be preset as the thresholdΔI₁. The “unit time”, for example, can be set arbitrarily in the rangeof 1 to 5 seconds. The amount of change per unit time in the currentflowing in the PTC heater 140 is calculated by the current detector 141,and when the calculated amount of change exceeds the threshold ΔI₁, theactivation of the expander 103 or the low-pressure stage compressor 105is detected. The PTC heater 140 and the current detector 141 can be usedalso in other embodiments and modifications.

<Operation of Refrigeration Cycle Apparatus 200>

FIG. 4 is a flow chart showing the activation control of therefrigeration cycle apparatus 200. When an activation command is inputin step S21, the controller 117 transmits control signals to theactuators of the expansion valve 110, the injection flow-regulatingvalve 112, and the bypass valve 202 so that the expansion valve 110 andthe injection flow-regulating valve 112 are fully opened as well as thebypass valve 202 is opened to a specific degree (step S22). Here, thephrase “the bypass valve 202 is opened to a specific degree” means to beset within a range of the degree of opening that allows the pressuredifference between the suction port and the discharge port of theexpander 103 to be maintained to the level that is required for theactivation of the expander 103. This “specific degree of opening” can bedetermined experimentally or theoretically. In short, the bypass valve202 is slightly opened so as to prevent excessive reduction in thepressure difference between before and after the expander 103.

Next, the low-pressure stage compressor 105 and the high-pressure supplypassage 130 are connected by controlling the three-way valve 133 (stepS23).

Next, the controller 117 starts to supply power to the motor 101 b inorder to activate the high-pressure stage compressor 101 (step S24).This activates the high-pressure stage compressor 101 and causes therefrigerant present in the intermediate-pressure flow passage 106 f, theinjection flow passage 111, the flow passage 106 c, the gas-liquidseparator 108, the flow passage 106 d, the evaporator 104, and a part ofthe flow passage 106 e (portion between the evaporator 104 and thethree-way valve 133) to be drawn into the high-pressure stage compressor101.

When the drawing of the refrigerant into the high-pressure stagecompressor 101 is started, as has been described in Embodiment 1 withreference to FIG. 6, the pressure at the suction port of each of theexpander 103 and the low-pressure stage compressor 105 is renderedrelatively high, and the pressure at the discharge port of each of theexpander 103 and the low-pressure stage compressor 105 is renderedrelatively low. As a result, the power recovery system 109 isself-activated smoothly.

Upon detecting the activation of the low-pressure stage compressor 105through the activation detector 119 (step S25), the controller 117controls the three-way valve 133 so that the low-pressure stagecompressor 105 and the evaporator 104 are connected (step S26). Thisallows the refrigerant to be supplied from the evaporator 104 to thelow-pressure stage compressor 105 through the flow passage 106 e. Theopening degree of each of the expansion valve 110 and the injectionflow-regulating valve 112 is adjusted because of the same reason as inEmbodiment 1 (step S27). Further, the bypass valve 202 is closed.Thereafter, transfer to the regular operation is performed.

<Effects of the Refrigeration Cycle Apparatus 200>

According to this modification, the following effects can be obtained inaddition to the effects described in Embodiment 1. According to thismodification, the controller 117 opens the bypass valve 202, before theactivation of the expander 103 and the low-pressure stage compressor105, to a degree within the range that allows a pressure differencerequired for the activation of the expander 103 to be produced betweenthe suction port and the discharge port of the expander 103. That is,the activation of the power recovery system 109 is attempted while thebypass valve 202 is slightly opened. The controller 117 closes thebypass valve 202 after the activation of the expander 103 and thelow-pressure stage compressor 105. Thus, a sudden reduction in thepressure difference between before and after the expander 103 can beprevented from occurring immediately after the activation of the powerrecovery system 109. Accordingly, smooth transfer to the regularoperation can be achieved while a driving force to continue theoperation of the power recovery system 109 is sufficiently ensured.

Modification 2

FIG. 5 is a configuration diagram of a refrigeration cycle apparatus 300in Modification 2. As shown in FIG. 5, the refrigeration cycle apparatus300 differs from Embodiment 1 in that a temperature detector thatdetects the temperature of the refrigerant at the discharge port of thelow-pressure stage compressor 105 is used as the activation detector119. In this modification, the same components as those in Embodiment 1are denoted by the same reference numerals, and the detaileddescriptions thereof are omitted.

In the case of using a temperature detector as the activation detector119, a threshold T₂ that has been determined experimentally ortheoretically, for example, is preset in the controller 117. When avalue obtained by subtracting a temperature that has been detected bythe temperature detector at a time going back for a unit time from thecurrent temperature detected by the temperature detector exceeds thespecific threshold T₂, the activation of the expander 103 or thelow-pressure stage compressor 105 is detected.

The temperature of the refrigerant at the discharge port of thelow-pressure stage compressor 105 is low during the period after theactivation of the high-pressure stage compressor 101 and before theactivation of the expander 103. When the low-pressure stage compressor105 starts to operate, the temperature of the refrigerant at thedischarge port of the low-pressure stage compressor 105 suddenlyincreases. The change in the temperature of the refrigerant at thedischarge port of the low-pressure stage compressor 105, for example, isabout 10° C., though it depends also on the intended use, operationalconditions, etc., of the refrigeration cycle apparatus 100. By capturingthis temperature change, the activation of the expander 103 or thelow-pressure stage compressor 105 can be detected. Specifically, thetemperature T of the refrigerant at the discharge port of thelow-pressure stage compressor 105 is detected per unit time and storedin the memory of the controller 117. Then, the last temperature T_(n)(n: natural number) previously stored in the memory and the currenttemperature T_(n+1) are compared. When the current temperature T_(n+1)significantly exceeds the last past temperature T_(n), in other words,when (T_(n+1)−T_(n))>T₂ is satisfied, the expander 103 or thelow-pressure stage compressor 105 can be determined to have beenactivated. It should be noted that the “unit time” can be set to asufficient time to capture the sudden reduction in the temperature T,for example, can be arbitrarily set in the range of 1 to 5 seconds.

Upon the activation of the low-pressure stage compressor 105, therefrigerant at high pressure and high temperature is drawn into thelow-pressure stage compressor 105 from the high-pressure supply passage130. Since the pressure in the flow passage 106 f is low, thelow-pressure stage compressor 105 temporarily functions as an expander.The refrigerant that has been expanded in the low-pressure stagecompressor 105 is discharged into the flow passage 106 f. Therefrigerant that has been compressed in the high-pressure stagecompressor 101 and that has been expanded again in the low-pressurestage compressor 105 obtains an enthalpy corresponding to the lossoccurring in each of the high-pressure stage compressor 101 and thelow-pressure stage compressor 105. That is, the refrigerant present inthe flow passage 106 f flows through the high-pressure stage compressor101 and the low-pressure stage compressor 105, and when it returns tothe flow passage 106 f again, the temperature of the refrigerantincreases by the increment in the enthalpy of the refrigerant. Thetemperature detector detects the activation of the low-pressure stagecompressor 105 by comparing the temperature increase with the thresholdT₂.

<Effects of Refrigeration Cycle Apparatus 300>

According to this modification, the following effects can be obtained inaddition to the effects described in Embodiment 1. In this modification,the activation is detected on the basis of the temperature of therefrigerant at the discharge port of the low-pressure stage compressor105. This can ensure the capture of the activation of the power recoverysystem 109, thereby enabling rapid transfer to the regular operation.

Embodiment 2

<Configuration of Refrigeration Cycle Apparatus 400>

FIG. 8 is a configuration diagram of a refrigeration cycle apparatus 400in Embodiment 2. As shown in FIG. 8, the refrigeration cycle apparatus400 differs from Embodiment 1 in that the high-pressure supply passage130, the on-off valve 131, and the check valve 132 are omitted. In thisembodiment, the same components as those in Embodiment 1 are denoted bythe same reference numerals, and the detailed descriptions thereof areomitted.

In this embodiment, activation control that is different from that inEmbodiment 1 is performed. That is, the expansion valve 110 is fullyclosed in the activation of the refrigeration cycle apparatus 400. Thisallows the pressure at the suction port of the low-pressure stagecompressor 105 to be maintained at the pressure in a standby state(before the activation of the high-pressure stage compressor 101). Uponthe activation of the high-pressure stage compressor 101, a pressuredifference occurs between the suction port and the discharge port of theexpander 103. Likewise, a pressure difference occurs between the suctionport and the discharge port of the low-pressure stage compressor 105. Asa result, the power recovery system 109 is activated.

In the activation of the refrigeration cycle apparatus 400, theinjection flow-regulating valve 112 is fully opened or substantiallyfully opened. When the injection flow-regulating valve 112 is opened inthe activation, the high-pressure stage compressor 101 can draw therefrigerant present in the flow passage 106 c, the gas-liquid separator108, and a part of the flow passage 106 d. This enables the pressure onthe high-pressure side of the main refrigerant circuit 106 to increaserapidly. Particularly, since the gas-liquid separator 108 is provided inthis embodiment, it is possible to store a sufficient amount ofrefrigerant between the discharge port of the expander 103 and theexpansion valve 110 during stoppage.

In this embodiment, the controller 117 controls the expansion valve 110according to a detection result of the activation detector 119.Specifically, it fully closes the expansion valve 110 in the activationof the refrigeration cycle apparatus 400. This can prevent the pressureat the suction port of the low-pressure stage compressor 105 from beingequal to the pressure at the discharge port of the low-pressure stagecompressor 105 via the injection flow passage 111. On the other hand,the controller 117 opens the expansion valve 110 after the activation ofthe expander 103 and the low-pressure stage compressor 105. For example,upon the reception of signals that indicate the activation of thelow-pressure stage compressor 105 from the activation detector 119, thecontroller 117 fully opens the expansion valve 110.

Also in this embodiment, the activation of the power recovery system 109can be detected by the method described in Embodiment 1. The control ofthe expansion valve 110 can be switched from the control before theactivation to the control after the activation, according to thedetection result. In this way, after the expansion valve 110 is opened,the power recovery system 109 continues to operate stably.

In this embodiment, a temperature detector that detects the temperatureof the refrigerant in a portion of the main refrigerant circuit 106 fromthe expansion valve 110 to the suction port of the low-pressure stagecompressor 105 (a part of the flow passage 106 d, the evaporator 104,and the flow passage 106 e) can be further used as the activationdetector 119. In this case, when the difference between a temperature ina standby state (before the activation of the high-pressure stagecompressor 101) detected by the temperature detector and the currenttemperature detected by the temperature detector exceeds a specificthreshold T₀, the activation of the expander 103 or the low-pressurestage compressor 105 is detected. Typically, a temperature detector thatdetects the evaporation temperature of the refrigerant in the evaporator104 can be used as the activation detector 119.

Likewise, a pressure detector that detects the pressure of therefrigerant in a portion of the main refrigerant circuit 106 from theexpansion valve 110 to the suction port of the low-pressure stagecompressor 105 can be used as the activation detector 119. When thedifference between a pressure in a standby state detected by thepressure detector and the current pressure detected by the pressuredetector exceeds a specific threshold P₀, the activation of the expander103 or the low-pressure stage compressor 105 is detected.

When the power recovery system 109 is activated, the low-pressure stagecompressor 105 draws the refrigerant present in the evaporator 104. Thiscauses a reduction in the temperature and the pressure in the evaporator104. An optimal threshold T₀ or threshold P₀ determined by anexperimental or theoretical technique is preset in the controller 117.The activation of the power recovery system 109 can be detected bycomparing the temperature change in flow passages from the expansionvalve 110 to the suction port of the low-pressure stage compressor 105(flow passages on the low-pressure side) with the threshold T₀.Likewise, the activation of the power recovery system 109 can bedetected by comparing the pressure change in flow passages on thelow-pressure side with the threshold P₀.

Also in this embodiment, the method for detecting the activation of theexpander 103 or the low-pressure stage compressor 105 is not limited toone, and a plurality of methods can be performed in combination. Forexample, the activation of the expander 103 or the low-pressure stagecompressor 105 is accurately captured by a method of monitoring thetemperature or pressure of the refrigerant in a portion of the mainrefrigerant circuit 106 from the expansion valve 110 to the suction portof the low-pressure stage compressor 105. Thereafter, whether the powerrecovery system 109 is in a state that allows continuous operation isdetermined by a method of comparing the temperature difference ΔT withthe threshold T₁, a method of comparing the pressure difference ΔP withthe threshold P₁, or a method of comparing the elapsed time t with thethreshold time t₁. When these plurality of conditions are satisfied, theexpander 103 or the low-pressure stage compressor 105 is determined tohave been activated, and the expansion valve 110 is opened.

<Operation of Refrigeration Cycle Apparatus 400>

FIG. 9 is a flow chart showing the activation control of therefrigeration cycle apparatus 400. The refrigeration cycle apparatus 400starts the regular operation after performing the activation controlshown in FIG. 9. In an operation standby state, the high-pressure stagecompressor 101 is stopped, the expansion valve 110 and the injectionvalve 112 are opened, and the pressure of the refrigerant in the mainrefrigerant circuit 106 is substantially uniform.

When an activation command is input in step ST11, the controller 117transmits control signals to the actuator of the expansion valve 110 sothat the expansion valve 110 is closed (fully closed) (step ST12).

Next, the controller 117 starts to supply power to the motor 101 b inorder to activate the high-pressure stage compressor 101 (step ST13).This activates the high-pressure stage compressor 101 and causes therefrigerant present in the intermediate-pressure flow passage 106 f, theinjection flow passage 111, the flow passage 106 c, the gas-liquidseparator 108, and a part of the flow passage 106 d (portion between thegas-liquid separator 108 and the expansion valve 110) to be drawn intothe high-pressure stage compressor 101. Instead of closing the expansionvalve 110 before the activation of the high-pressure stage compressor101, it is also possible to close the expansion valve 110 correspondingto the activation of the high-pressure stage compressor 101.

A fan or pump that causes a fluid (air or water) for heat exchange withthe refrigerant to flow into the heat radiator 102 is activated,corresponding to the activation of the high-pressure stage compressor101. This can prevent an excessive increase in the high pressure of thecycle. The fan or pump of the evaporator 104 may be activatedcorresponding to the activation of the high-pressure stage compressor101, or may be activated after the expansion valve 110 is opened. Inorder to maintain the pressure at the suction port of the low-pressurestage compressor 105 to the pressure in a standby state, the latteroperation is recommended.

Once the drawing of the refrigerant into the high-pressure stagecompressor 101 is started, the internal pressure of theintermediate-pressure flow passage 106 f, etc., decreases. On the otherhand, since the refrigerant compressed in the high-pressure stagecompressor 101 is discharged, the pressure in flow passages from thedischarge port of the high-pressure stage compressor 101 to the suctionport of the expander 103 (the flow passage 106 a, the heat radiator 102,and the flow passage 106 b) increases. Meanwhile, the pressure of therefrigerant in flow passages from the expansion valve 110 to the suctionport of the low-pressure stage compressor 105 (a part of the flowpassage 106 d, the evaporator 104, and the flow passage 106 e) ismaintained to the pressure in the refrigerant circuit 106 duringstoppage of the refrigeration cycle apparatus 400.

As a result, as shown in FIG. 14A, a pressure difference can be producednot only between the suction port and the discharge port of the expander103, but also between the suction port and the discharge port of thelow-pressure stage compressor 105. The pressure difference of therefrigerant acts on each of the expander 103 and the low-pressure stagecompressor 105, and thus self-activation of the power recovery system109 can be achieved easily. The high-pressure stage compressor 101 candraw a sufficient amount of the refrigerant to produce a large pressuredifference because the injection flow passage 111 and the gas-liquidseparator 108 are provided.

Upon detecting the activation of the low-pressure stage compressor 105through the activation detector 119 (step ST14), the controller 117transmits control signals to the actuator of the expansion valve 110 sothat the expansion valve 110 is fully opened (or substantially fullyopened) (step ST15). This causes the gas-liquid two-phase refrigerantwhose pressure has been reduced in the expander 103 to be supplied tothe gas-liquid separator 108. After the completion of the activationcontrol shown in FIG. 9, transfer to the regular operation where therefrigerant is circulated in the main refrigerant circuit 106 isperformed in the refrigeration cycle apparatus 400. In the regularoperation, the opening degree of each of the expansion valve 110 and theinjection flow-regulating valve 112 is adjusted so that excess supply ofthe liquid refrigerant to the high-pressure stage compressor 101 throughthe injection flow passage 111 and the flow passage 106 f is prevented.

Also in this embodiment, the operation of the refrigeration cycleapparatus 400 can be stopped according to the method described inEmbodiment 1.

<Effects of Refrigeration Cycle Apparatus 400>

According to this embodiment, the high-pressure stage compressor 101 candraw and compress the refrigerant in the gas-liquid separator 108 in theactivation of the refrigeration cycle apparatus 400. Therefore, thepressure in flow passages from the discharge port of the high-pressurestage compressor 101 to the suction port of the expander 103 can beincreased rapidly. Since a large pressure difference is produced betweenthe suction port and the discharge port of the expander 103, the powerrecovery system 109 is self-activated smoothly.

Further, it is possible to maintain the pressure of the refrigerant inflow passages from the expansion valve 110 to the suction port of thelow-pressure stage compressor 105 to the pressure in the refrigerantcircuit 106 during stoppage of the refrigeration cycle apparatus 400 byclosing the expansion valve 110. Therefore, a pressure difference occursalso between the suction port and the discharge port of the low-pressurestage compressor 105. This fact contributes to a smootherself-activation of the power recovery system 109. In this embodiment,the low-pressure stage compressor 105 and the expander 103 each have acertain suction volume. Particularly, when the suction volume of thelow-pressure stage compressor 105 is larger than the suction volume ofthe expander 103, the power recovery system 109 is activated moresmoothly by producing a pressure difference between the suction port andthe discharge port of the low-pressure stage compressor 105.

In the case where a specific condition is satisfied without detectingthe activation of the expander 103 or the low-pressure stage compressor105 after the activation of the high-pressure stage compressor 101, itcan be determined that the activation of the power recovery system 109has been failed. When the activation of the power recovery system 109has been failed, the controller 117 stops the high-pressure stagecompressor 101 and performs the control to activate the power recoverysystem 109 again. That is, when a failure of the activation is detected,the expansion valve 110 is once fully opened. Thereafter, the activationcontrol described with reference to FIG. 9 is performed. In this way, itis possible to prevent an excessive increase in the pressure in flowpassages from the discharge port of the high-pressure stage compressor101 to the suction port of the expander 103. It is also possible toprevent damages to the components of the expander 103 from occurring dueto an excessive pressure difference between before and after theexpander 103. Thus, the reliability of the refrigeration cycle apparatus400 can be improved.

The method for detecting a failure in the activation of the powerrecovery system 109 is not specifically limited. For example, after theactivation of the high-pressure stage compressor 101, the currenttemperature (or pressure) of the refrigerant in flow passages from theexpansion valve 110 to the suction port of the low-pressure stagecompressor 105 (flow passages on the low-pressure side), e.g., in theevaporator 104 is detected. When the difference between the detectedtemperature (or pressure) and a reference temperature (or referencepressure) does not reach a specific threshold within a certain period oftime, the power recovery system 109 can be determined to have failed tobe activated. As the threshold, the aforementioned threshold T₀ orthreshold P₀ can be used. As the reference temperature (or referencepressure), the temperature (or pressure) of the refrigerant in theevaporator 104 before the activation of the high-pressure stagecompressor 101 can be used. In the case where a certain period of timehas elapsed without detecting the activation of the power recoverysystem 109 after the activation of the high-pressure stage compressor101, it can be determined that the activation of the power recoverysystem 109 has been failed. It is also possible to determine whether ornot the activation of the power recovery system 109 has been failed bydetecting the temperature or pressure in flow passages from thedischarge port of the high-pressure stage compressor 101 to the suctionport of the expander 103 (flow passages on the high-pressure side), onthe basis of the difference between the detected temperature or pressureand the temperature or pressure in the flow passages on thehigh-pressure side before the activation of the high-pressure stagecompressor 101.

Modification 3

<Configuration of Refrigeration Cycle Apparatus 500>

FIG. 10 is a configuration diagram of a refrigeration cycle apparatus500 in Modification 3. As shown in FIG. 10, the refrigeration cycleapparatus 500 is provided with the bypass flow passage 201 and thebypass valve 202. Other configurations are the same as those inEmbodiment 2. In this modification, the same components as those inEmbodiment 2 are denoted by the same reference numerals, and thedetailed descriptions thereof are omitted.

The bypass flow passage 201 is connected to the main refrigerant circuit106 so as to bypass the expander 103. The upstream end E₃ of the bypassflow passage 201 is located on the flow passage 106 b, and thedownstream end E₄ thereof is located on the flow passage 106 c. Thebypass valve 202 is provided on the bypass flow passage 201. The bypassflow passage 201, typically, is constituted by a refrigerant pipe. Asthe bypass valve 202, a valve, which allows the degree of opening to bevaried stepwise, capable of expanding a refrigerant, typically, anelectric expansion valve is preferably used.

The element portion of the activation detector 119 is provided on theflow passage 106 b. The element portion of the activation detector 119may be located on the heat radiator 102 side, or may be located on theexpander 103 side, as seen from the upstream end E₃ of the bypass flowpassage 201.

<Operation of Refrigeration Cycle Apparatus 500>

FIG. 11 is a flow chart showing the activation control of therefrigeration cycle apparatus 500. When an activation command is inputin step ST21, the controller 117 transmits control signals to theactuators of the expansion valve 110 and the bypass valve 202 so thatthe expansion valve 110 is fully closed, and the bypass valve 202 isopened to a specific degree (step ST22). Here, the phrase “the bypassvalve 202 is opened to a specific degree” means to be set within a rangeof the degree of opening that allows the pressure difference between thesuction port and the discharge port of the expander 103 to be maintainedto the level that is required for the activation of the expander 103.This “specific degree of opening” can be determined experimentally ortheoretically. In short, the bypass valve 202 is slightly opened so asto prevent excessive reduction in the pressure difference between beforeand after the expander 103.

Next, the controller 117 starts to supply power to the motor 101 b inorder to activate the high-pressure stage compressor 101 (step ST23).This activates the high-pressure stage compressor 101 and causes therefrigerant present in the intermediate-pressure flow passage 106 f, theinjection flow passage 111, the flow passage 106 c, the gas-liquidseparator 108, and a part of the flow passage 106 d to be drawn into thehigh-pressure stage compressor 101.

When the drawing of the refrigerant into the high-pressure stagecompressor 101 is started, as has been described in Embodiment 2 withreference to FIG. 14A, a pressure difference can be produced not onlybetween the suction port and the discharge port of the expander 103, butalso between the suction port and the discharge port of the low-pressurestage compressor 105. The pressure difference of the refrigerant acts oneach of the expander 103 and the low-pressure stage compressor 105, andthus self-activation of the power recovery system 109 can be achievedeasily.

Upon detecting the activation of the low-pressure stage compressor 105through the activation detector 119 (step ST24), the controller 117transmits control signals to the actuator of the expansion valve 110 sothat the expansion valve 110 is fully opened (or substantially fullyopened) (step ST25). Further, it transmits control signals to theactuator of the bypass valve 202 so that the bypass valve 202 is fullyclosed.

<Effects of Refrigeration Cycle Apparatus 500>

According to this modification, the following effects can be obtained inaddition to the effects described in Embodiment 2. According to thismodification, the controller 117 opens the bypass valve 202, before theactivation of the expander 103 and the low-pressure stage compressor105, to a degree within the range that allows a pressure differencerequired for the activation of the expander 103 to be produced betweenthe suction port and the discharge port of the expander 103. That is,the activation of the power recovery system 109 is attempted while thebypass valve 202 is slightly opened. The controller 117 closes thebypass valve 202 after the activation of the expander 103 and thelow-pressure stage compressor 105. Thus, a sudden reduction in thepressure difference between before and after the expander 103 can beprevented from occurring immediately after the activation of the powerrecovery system 109. Accordingly, smooth transfer to the regularoperation can be achieved while a driving force to continue theoperation of the power recovery system 109 is sufficiently ensured.

Modification 4

<Configuration of Refrigeration Cycle Apparatus 600>

FIG. 12 is a configuration diagram of a refrigeration cycle apparatus600 in Modification 4. As shown in FIG. 12, the refrigeration cycleapparatus 600 is further provided with a bypass flow passage 301 and abypass valve 302. Other configurations are the same as in Embodiment 2.In this modification, the same components as those in Embodiment 2 aredenoted by the same reference numerals, and the detailed descriptionsthereof are omitted.

The bypass flow passage 301 is connected to the main refrigerant circuit106 so as to communicate the flow passage 106 b and the flow passage 106d. The bypass valve 302 is provided on the bypass flow passage 301 andcontrols the flow of the refrigerant in the bypass flow passage 301. Thebypass flow passage 301, typically, is constituted by a refrigerantpipe. As the bypass valve 302, an on-off valve can be used.

Specifically, the bypass flow passage 301 has an upstream end E₅ locatedat a portion of the main refrigerant circuit 106 from the outlet of theheat radiator 102 to the suction port of the expander 103 (the flowpassage 106 b), and a downstream end E₆ located at a portion of the mainrefrigerant circuit 106 from the expansion valve 110 to the inlet of theevaporator 104 (a part of the flow passage 106 d). The bypass flowpassage 301 allows the refrigerant at high pressure in the flow passage106 b to be introduced directly to the suction port of the low-pressurestage compressor 105.

As long as the pressure at the suction port of the low-pressure stagecompressor 105 can be increased, the positions of the upstream end E₅and the downstream end E₆ are not limited to the positions shown in FIG.12. That is, as long as the portion of the main refrigerant circuit 106from the discharge port of the high-pressure stage compressor 101 to thesuction port of the expander 103 and a portion of the main refrigerantcircuit 106 from the expander 110 to the suction port of thelow-pressure stage compressor 105 can be communicated, the position ofthe upstream end E₅ is not specifically limited. Specifically, thebypass flow passage 301 may be connected to the main refrigerant circuit106 so as to communicate the flow passage 106 a and the flow passage 106e. Depending on the circumstances, the bypass flow passage 301 maybranch from the heat radiator 102. For example, in the case where theheat radiator 102 is constituted by an upstream part and a downstreampart, the bypass flow passage 301 can easily branch from a portionbetween these two parts.

<Operation of Refrigeration Cycle Apparatus 600>

FIG. 13 is a flow chart showing the activation control of therefrigeration cycle apparatus 600. When an activation command is inputin step ST31, the controller 117 transmits control signals to theactuators of the expansion valve 110 and the bypass valve 302 so thatthe expansion valve 110 is fully closed, and the bypass valve 302 isfully opened (step ST32).

Next, the controller 117 starts to supply power to the motor 101 b inorder to activate the high-pressure stage compressor 101 (step ST33).This activates the high-pressure stage compressor 101 and causes therefrigerant present in the intermediate-pressure flow passage 106 f, theinjection flow passage 111, the flow passage 106 c, the gas-liquidseparator 108, and a part of the flow passage 106 d to be drawn into thehigh-pressure stage compressor 101.

When the drawing of the refrigerant into the high-pressure stagecompressor 101 is started, as shown in FIG. 14B, a large pressuredifference can be produced not only between the suction port and thedischarge port of the expander 103, but also between the suction portand the discharge port of the low-pressure stage compressor 105. Thepressure difference of the refrigerant acts on each of the expander 103and the low-pressure stage compressor 105, and thus self-activation ofthe power recovery system 109 can be achieved easily. Particularly,according to this modification, the pressure at the suction port of thelow-pressure stage compressor 105 can be increased due to the functionsof the bypass flow passage 301 and the bypass valve 302.

Upon detecting the activation of the low-pressure stage compressor 105through the activation detector 119 (step ST34), the controller 117transmits control signals to the actuator of the expansion valve 110 sothat the expansion valve 110 is fully opened (or substantially fullyopened) (step ST35). Further, it transmits control signals to theactuator of the bypass valve 302 so that the bypass valve 302 is fullyclosed.

<Effects of Refrigeration Cycle Apparatus 600>

According to this modification, the following effects can be obtained inaddition to the effects described in Embodiment 2. According to thismodification, the pressure at the suction port of the low-pressure stagecompressor 105 can be also increased through the bypass flow passage301. Accordingly, the drive torque to be imparted to the low-pressurestage compressor 105 increases, and a smoother activation of the powerrecovery system 109 is enabled.

Various constituents described in the respective embodiments andmodifications can be applied to other embodiments and modificationswithout restriction as long as no technical contradiction occurs. Forexample, the three-way valve 133 (see FIG. 3) described in Modification2 can be applied to Embodiment 1 and Modification 2.

INDUSTRIAL APPLICABILITY

The refrigeration cycle apparatus of the present invention is useful fordevices such as water heaters, air conditioners, and dryers.

1. A refrigeration cycle apparatus comprising: a main refrigerantcircuit having a low-pressure stage compressor that compresses arefrigerant, a high-pressure stage compressor that further compressesthe refrigerant that has been compressed in the low-pressure stagecompressor, a heat radiator that cools the refrigerant that has beencompressed in the high-pressure stage compressor, an expander thatrecovers power from the refrigerant that has been cooled in the heatradiator while expanding the refrigerant, the expander being coupled tothe low-pressure stage compressor by a shaft so that the recovered poweris transferred to the low-pressure stage compressor, a gas-liquidseparator that separates the refrigerant that has been expanded in theexpander into a gas refrigerant and a liquid refrigerant, and anevaporator that evaporates the liquid refrigerant that has beenseparated in the gas-liquid separator; an injection flow passage thatintroduces the gas refrigerant that has been separated in the gas-liquidseparator into a portion of the main refrigerant circuit from adischarge port of the low-pressure stage compressor to a suction port ofthe high-pressure stage compressor; a high-pressure supply passage thatcommunicates a portion of the main refrigerant circuit from a dischargeport of the high-pressure stage compressor to a suction port of theexpander and a portion of the main refrigerant circuit from an outlet ofthe evaporator to a suction port of the low-pressure stage compressor; aflow passage-switching mechanism capable of selectively connecting oneselected from the evaporator and the high-pressure supply passage to thelow-pressure stage compressor so that the refrigerant is introduced fromthe evaporator or the high-pressure supply passage to the low-pressurestage compressor; and a controller that controls the flowpassage-switching mechanism before activation of the expander and thelow-pressure stage compressor so that the refrigerant is introduced fromthe high-pressure supply passage to the low-pressure stage compressor,while controlling the flow passage-switching mechanism after theactivation of the expander and the low-pressure stage compressor so thatthe refrigerant is introduced from the evaporator to the low-pressurestage compressor, wherein a discharge pressure of the high-pressurestage compressor is applied to the suction port of the low-pressurestage compressor and the suction port of the expander while a suctionpressure of the high-pressure stage compressor is applied to thedischarge port of the low-pressure stage compressor and a discharge portof the expander before activation of the expander and the low-pressurestage compressor.
 2. The refrigeration cycle apparatus according toclaim 1, wherein the high-pressure supply passage has an upstream endconnected to a portion of the main refrigerant circuit from thedischarge port of the high-pressure stage compressor to an inlet of theheat radiator.
 3. The refrigeration cycle apparatus according to claim1, wherein the high-pressure supply passage has a downstream endconnected to a portion of the main refrigerant circuit from the outletof the evaporator to the suction port of the low-pressure stagecompressor, and the flow passage-switching mechanism is constituted byan on-off valve provided on the high-pressure supply passage and avalve, provided in a portion of the main refrigerant circuit from theoutlet of the evaporator to the downstream end of the high-pressuresupply passage, capable of blocking flow of the refrigerant from thehigh-pressure supply passage toward the evaporator.
 4. The refrigerationcycle apparatus according to claim 1, wherein the high-pressure supplypassage has a downstream end connected to a portion of the mainrefrigerant circuit from the outlet of the evaporator to the suctionport of the low-pressure stage compressor, and the flowpassage-switching mechanism is constituted by a three-way valve providedat the downstream end of the high-pressure supply passage.
 5. (canceled)6. The refrigeration cycle apparatus according to claim 1, furthercomprising: an activation detector that detects the activation of theexpander or the low-pressure stage compressor, wherein the controllerswitches control of the flow passage-switching mechanism from controlbefore the activation to control after the activation, according to adetection result of the activation detector.
 7. The refrigeration cycleapparatus according to claim 6, wherein the activation detector includesa timer that measures an elapsed time from activation of thehigh-pressure stage compressor, and when the time measured by the timerexceeds a specific threshold time, the activation of the expander or thelow-pressure stage compressor is detected.
 8. The refrigeration cycleapparatus according to claim 6, wherein the activation detector includesa temperature detector that detects a difference between a temperatureof the refrigerant at the suction port of the expander and a temperatureof the refrigerant at the discharge port of the expander, and when thetemperature difference detected by the temperature detector exceeds aspecific threshold, activation of the expander or the low-pressure stagecompressor is detected.
 9. The refrigeration cycle apparatus accordingto claim 6, wherein the activation detector includes a pressure detectorthat detects a difference between a pressure of the refrigerant at thesuction port of the expander and a pressure of the refrigerant at thedischarge port of the expander, and when the pressure differencedetected by the pressure detector exceeds a specific threshold,activation of the expander or the low-pressure stage compressor isdetected.
 10. The refrigeration cycle apparatus according to claim 6,wherein the activation detector includes a pressure detector thatdetects a difference between a pressure of the refrigerant at thesuction port of the expander and a pressure of the refrigerant at thedischarge port of the expander, and when a value obtained by subtractinga current pressure difference detected by the pressure detector from apressure difference that has been detected by the pressure detector at atime going back for a unit time exceeds a specific threshold, activationof the expander or the low-pressure stage compressor is detected. 11.The refrigeration cycle apparatus according to claim 6, wherein theactivation detector includes a temperature detector that detects adifference between a temperature of the refrigerant at the suction portof the expander and a temperature of the refrigerant at the dischargeport of the expander, and when a value obtained by subtracting a currenttemperature difference detected by the temperature detector from atemperature difference that has been detected by the temperaturedetector at a time going back for a unit time exceeds a specificthreshold, activation of the expander or the low-pressure stagecompressor is detected.
 12. The refrigeration cycle apparatus accordingto claim 6, wherein the activation detector includes a PTC heaterprovided in a portion from an outlet of the heat radiator to the suctionport of the expander of the main refrigerant circuit, and when theamount of change per unit time in an electric current flowing in the PTCheater exceeds a specific threshold, activation of the expander or thelow-pressure stage compressor is detected.
 13. The refrigeration cycleapparatus according to claim 6, wherein the activation detector is atemperature detector that detects a temperature of the refrigerant atthe discharge port of the low-pressure stage compressor, and when avalue obtained by subtracting a temperature that has been detected bythe temperature detector at a time going back for a unit time from acurrent temperature detected by the temperature detector exceeds aspecific threshold, activation of the expander or the low-pressure stagecompressor is detected.
 14. The refrigeration cycle apparatus accordingto claim 1, wherein in the case where activation of the expander and thelow-pressure stage compressor has been failed, the controller stops thehigh-pressure stage compressor and performs control to activate theexpander and the low-pressure stage compressor again.
 15. Therefrigeration cycle apparatus according to claim 1, wherein the expanderand the low-pressure stage compressor are accommodated in a singleclosed casing.
 16. The refrigeration cycle apparatus according to claim1, further comprising: a bypass flow passage that bypasses the expander;and a bypass valve provided on the bypass flow passage, wherein thecontroller opens the bypass valve to a specific degree before theactivation of the expander and the low-pressure stage compressor andcloses the bypass valve after the activation of the expander and thelow-pressure stage compressor.
 17. The refrigeration cycle apparatusaccording to claim 1, wherein the expander and the low-pressure stagecompressor each have a certain suction volume, and the suction volume ofthe low-pressure stage compressor is larger than the suction volume ofthe expander. 18-30. (canceled)